In recent years, semiconductor lasers using a III-V group nitride compound semiconductor such as AlInGaN have been actively studied, developed and put to practical use as a semiconductor laser capable of light emission from a blue region to an ultraviolet region necessary for improving the density of optical disks, in place of conventional GaAs semiconductor lasers (see, Patent Document 1).
In such semiconductor lasers, an electron barrier layer having a bandgap larger than that of an active layer is inserted on the p-electrode side of the active layer to prevent an overflow of electrons from the active layer and hence a reduction in light emission efficiency. A p-type AlGaN or AlInGaN material is used as this electron barrier layer. If the Al composition ratio is higher, a bandgap is larger and the effect of restricting an overflow of electrons is increased. However, if the Al composition ratio is excessively high, the crystallinity is degraded to reduce the light emission efficiency instead of restricting the reduction in light emission efficiency. Therefore the Al composition ratio in the electron barrier layer is determined by considering this advantage and disadvantage. Also, if the thickness of the electron barrier layer is larger, the effect of restricting an overflow of electrons is increased. However, an adverse effect of degrading the crystallinity is produced in such a case. The layer thickness is determined by considering this advantage and disadvantage.
On the other hand, if the electron barrier layer has a thickness large enough to sufficiently restrict an overflow of electrons, it is advantageous, from the viewpoint of crystallinity, to reduce the Al composition ratio in a light guide layer and a clad layer on the p-electrode side of the electron barrier layer relative to that in the electron barrier layer. For this reason, the light guide layer and the p-type clad layer are formed of AlGaN, GaN or InGaN having an Al composition ratio lower than that in the electron barrier layer. FIG. 6 is a sectional view of a conventional semiconductor light emitting device having such a structure.
As shown in FIG. 6, an n-type buffer layer 2 formed of GaN and having a thickness of 1.0 μm, an n-type clad layer 3 formed of AlGaN and having a thickness of 1.0 μm and Al composition ratio of 0.07, an n-type light guide layer 4 formed of GaN and having a thickness of 100 nm, an optical waveguide layer 5 formed of undoped InGaN and having a thickness of 7 nm, an active layer 6, an optical waveguide layer 7 formed of undoped InGaN and having a thickness of 20 nm and an In composition ratio of 0.02, an electron barrier layer 8 formed of p-type AlGaN and having a thickness of 20 nm and an Al composition ratio of 0.2, a p-type light guide layer 9 formed of p-type GaN and having a thickness of 100 nm, a p-type clad layer 10 formed of AlGaN and having a thickness of 400 nm and an Al composition ratio of 0.07 and a p-type contact layer 11 formed of GaN and having a thickness of 100 nm are successively formed in this order from the bottom on a GaN substrate 1.
The active layer 6 is a multiple quantum well structure in which three undoped InGaN well layers each having a thickness of 3.5 nm and an In composition ratio of 0.14 and two undoped InGaN barrier layers each having a thickness of 7.0 nm and an In composition ratio of 0.02 are alternately stacked. In the p-type clad layer 10 and the p-type contact layer 11, a ridge 12 is formed in the <1100> orientation by etching. The width of the ridge 12 is 1.5 μm and the etching depth is 450 nm. An SiO2 insulating film 13 having a thickness of 200 nm is formed so as to cover side surface portions of the ridge 12 and the upper surface of the p-type clad layer 10. An opening 14 is provided in the insulating film 13 above the ridge 12. A p-type electrode 15 electrically contacts the p-type contact layer 11 through the opening 14. The p-type electrode 15 is provided, for example, by successively stacking Pd and Au films. An n-type electrode 16 is provided on the back surface of the GaN substrate 1 by successively stacking Ti and Al films.
Patent Document 1: Japanese Patent Laid-Open No. 7-235725